Gas discharge lamps such as fluorescent lamps are most efficiently operated when driven with an AC voltage of high frequency, typically 30KHz. Such a drive voltage is typically generated by a resonant "tank" circuit made up of an inductive element and a capacitive element. The tank circuit is typically supplied from a utility mains (e.g. having voltage of 120VAC, 60Hz) via a rectifier and an inverter. The inverter typically includes series-connected transistors whose control electrodes are transformer-coupled to the tank circuit output so that the inverter provides to the tank circuit a supply which alternates at the frequency of the tank circuit.
In a known type of circuit for driving two or more fluorescent lamps, a series-resonant tank circuit is used. In such a resonant circuit the inductive element and the capacitive element are connected in series. Such a series-resonant circuit behaves most like a current source, i.e. at its resonant frequency it generates a signal whose current remains substantially constant, independent of the voltage supplied. To such a series-resonant circuit, a multiple fluorescent lamp load is typically connected with the lamps in series. Since a series-resonant circuit behaves most like a current source, such a series-resonant circuit is inherently self-ballasting and so does not require additional ballasting components. Such a series connection arrangement of lamps to a series-resonant circuit generates less power than older drive circuits arrangements (which employ parallel-resonant tank circuits driving parallel connected lamps), enabling lower-rated transformers and other components to be used, and wasting less energy through dissipation. Another advantage of using a series-resonant circuit to drive fluorescent lamps is that such a circuit automatically achieves a high voltage at power-on, which aids striking of the lamps.
Typically, in such a series-resonant circuit the inverter is coupled to the tank circuit output by a saturating-core transformer. The use of a saturating-core transformer enables rapid switching of the inverter transistors, allowing relatively tight control of the inverter output. However, such saturating core transformers are highly specified components which are typically expensive.